only Congress has constitutional authority to establish U.S. immigration policy, and fundamental reform requires legislative action. Thus the administration’s recent announcement that deportation will be sought only for undocumented immigrants who have committed crimes in the United States violates the separation of powers and is unconstitutional.

No president, of course, can hope to expel every undocumented person in the United States — they number perhaps upward of 11 million people. Human and financial resources to identify, apprehend, process and promptly deport millions have been lacking for years as has, arguably, the political will to do so. In this environment, immigration enforcement authorities, under administrations of both parties, have performed as best they could given their resources. Still, millions have been deported over the years. And while many had been convicted of serious criminal offenses, most deportees have not been in that category.

The policy that Obama unveiled last month differs fundamentally from the spotty immigration enforcement records of previous presidents. The administration indicated that, henceforth, deportation will be focused solely on illegal immigrants who have criminal records and that no enforcement resources will be expended on those who do not pose a threat to public safety. The effect is that undocumented individuals who have avoided apprehension at the border and not been convicted of a serious offense since arriving to the United States will no longer face the prospect of deportation, the most basic means of immigration enforcement.

This goes far beyond merely prioritizing the use of limited immigration enforcement resources. And it exceeds the president’s constitutional authority by, in effect, suspending operation of the immigration laws with respect to a very large and identifiable class of offenders.

Federal agencies establish enforcement priorities because Congress rarely votes adequate monies to fully implement any federal program. Law enforcement agencies such as Immigration and Customs Enforcement properly exercise prosecutorial discretion in deciding which offenses to investigate and prosecute. That discretion ultimately resides in the president. It allows him to establish priorities — properly informed by his policy preferences — on at least two levels.

First, a president can decide to devote more resources to a particular problem — human trafficking or white-collar crime, for example — with the result that other laws or areas of concern (say, organized crime) will be less vigorously pursued and enforced. This is entirely lawful and appropriate. Presidents are elected in part to establish such priorities.

Second, law enforcement officials must make determinations whether and how to direct their efforts in individual cases. Under the manual governing U.S. attorneys, for example, federal prosecutors must consider whether there is a sufficient federal interest before pursuing a case. This involves questions such as the nature and seriousness of an offense, potential deterrent effect on others, the defendant’s record, alternatives to criminal prosecution, likelihood of success and established law enforcement priorities.

This discretion is fully supported by the president’s constitutional authority. But that authority is not boundless. While the president can legitimately decide that, say, most FBI resources should be dedicated to the investigation and prosecution of terrorism cases, he cannot decree that no enforcement resources whatsoever should be allocated to securities fraud or counterfeiting cases. Since in our constitutional system the executive branch has exclusive license to enforce federal criminal laws, this would effectively decriminalize securities fraud and counterfeiting, vitiating the federal statutes that proscribed such activities.

In this regard, the Constitution provides that the president “shall take care that the laws be faithfully executed.” A century before the Constitution took effect, Britain’s King James II was deposed in no small part because he claimed the right to generally suspend laws enacted by Parliament and to dispense with law in individual cases.

James hoped to benefit his fellow Roman Catholics by issuing a “declaration of indulgence,” suspending operation of the religious penal laws Parliament had enacted against Roman Catholics and non-Anglican Protestants. The opposition in Britain was not merely due to religious bigotry. Acknowledging the power to suspend or dispense with laws, even when used to grant religious tolerance, would fatally warp any balance between executive and legislative authority. A legislature has no power to speak of if the executive can simply decide not to enforce its law. Thus, the suspending and dispensing powers were declared illegal in the English Bill of Rights. Knowing this history, the Framers gave the president no choice but to execute laws passed by Congress. As the Supreme Court stated in the 1838 case Kendall v. United States, the power to dispense with laws enacted by Congress “has no countenance for its support in any part of the constitution.”

Obama has not declared his intent to dispense with immigration law, and the point at which permissible executive enforcement discretion becomes suspension of statutory requirements often is one of degree. In this case, however, there is little question that the line has been crossed. The president is entitled to establish enforcement priorities, but the ultimate goal must always be implementation of the law as enacted by Congress. If the president disagrees with that law, he must persuade Congress to change it.

The writers are partners in the Washington office of Baker & Hostetler; they served in the Justice Department under presidents Ronald Reagan and George H.W. Bush.